Spectroscopy is the study of the interaction between electromagnetic radiation and a sample (e.g., containing one or more of a gas, solid and liquid). The manner in which the radiation interacts with a particular sample depends upon the properties (e.g., molecular composition) of the sample. Generally, as the radiation passes through the sample, specific wavelengths of the radiation are absorbed by molecules within the sample. The specific wavelengths of radiation that are absorbed are unique to each of the molecules within the specific sample. By identifying which wavelengths of radiation are absorbed, it is therefore possible to identify the specific molecules present in the sample.
Infrared spectroscopy is a particular field of spectroscopy in which, for example, the types of molecules and the concentration of individual molecules within a sample are determined by subjecting the sample (e.g., gas, solid, liquid or combination thereof) to infrared electromagnetic energy. Generally, infrared energy is characterized as electromagnetic energy having wavelengths of energy between about 0.7 μm (frequency 14,000 cm−1) and about 1000 μm (frequency 10 cm−1). Infrared energy is directed through the sample and the energy interacts with the molecules within the sample. The energy that passes through the sample is detected by a detector (e.g., an electromagnetic detector). The detected signal is then used to determine, for example, the molecular composition of the sample and the concentration of specific molecules within the sample.
One particular type of infrared spectrometer is the Fourier Transform Infrared (FTIR) spectrometer. They are used in a variety of industries, for example, air quality monitoring, explosive and biological agent detection, semiconductor processing, and chemical production. Different applications for FTIR spectrometers require different detection sensitivity to enable a user to distinguish between which molecules are present in a sample and to determine the concentration of the different molecules. In some applications, it is necessary to identify the concentration of individual molecules in a sample to within about one part per billion (ppb). As industrial applications require increasingly better sensitivity, optimization of existing spectroscopy systems and utilization of new spectroscopy components can enable the system to repeatably and reliably resolve smaller and smaller concentrations of molecules in samples.
FTIR spectrometers can also be used to monitor concentrations of compounds, e.g., in gases. Biofuels (e.g., biogas) are used to power various equipment, including turbine generators. The biogas is burned to power the equipment. Biogas (e.g., gas from animal waste, wastewater or a landfill) can include, a variety of compounds, including, siloxane compounds. Siloxane compounds in the biogas are also burned which creates oxides (e.g., SiO2 (e.g., silica, or sand)). The SiO2 can coat both the turbine blades as well as the turbine bearings, resulting in decreased performance or even failure of the turbine. The coating process is accelerated with higher levels of siloxane in the biogas. Biogas producers usually use an activated charcoal filter to trap the siloxanes, however, when the filter is expended the siloxane level rises.
Traditional methods for monitoring concentrations of siloxane compounds in a biogas are performed offline by analyzing samples taken from the biogas. For example, traditional techniques involve using GC/MS (i.e., gas chromatography/mass spectrometry) techniques to separate the siloxanes from the background gas and measure them. To analyze the sample gas, a sample is grabbed for analysis and run on the GC/MS system. A field sample is usually taken from the gas stream and introduced into either a stainless steel canister, a Tedlar sample bag or collected using a Methanol solvent impinger. This sample is then transported back to the analytical lab and analyzed; the analytical result is usually not known for days. Samples have the tendency to let components condense out which makes it hard to assess the true composition in the sample. Samples taken in this manner also only provide a single shot in time at which the contents are analyzed and therefore, may not be representative of the true composition of the sample. The GC/MS analysis of the sample can also take several hours to analyze the siloxane compounds in the sample which may be too late to allow for operator intervention. If a rise in siloxane levels had occurred, the opportunity to perform any actionable recourse may have already passed. Enhancing the ability to monitor and measure concentrations of siloxane in a biogas can enable greater turbine life. Furthermore, being able to monitor and quickly detect and quantify siloxane compounds can provide greater time for actionable recourse/intervention.